Passive displays, e.g., liquid crystal displays, digital micromirror device (DMD) displays, electrochromic displays, and so forth, typically modulate light that is provided from a light source, e.g., from a backlight, edge light, ambient light or some other light source. The description below is presented with respect to liquid crystal displays (sometimes referred to as LCD or as LCDs), although it will be appreciated that the description may apply similarly to other passive displays.
A problem encountered by passive displays occurs in bright ambient lighting conditions, which may make it difficult to interpret the image presented on a passive display. It may be difficult to “make out,” to understand, or to see details in an image that is shown on a passive display under bright ambient lighting conditions. The bright ambient light may be from bright sunlight, a brightly lit room or some other condition or environment in which there is bright light condition. This problem may occur without regard to the device or system in which the passive display is used; examples include portable computers, e.g., laptop, notebook and hand-size computers, portable digital assistants, mobile telephones, and hand-held games.
As only one example, the problem of reduced legibility in bright ambient light conditions may be due to reduced contrast between relatively bright portions and relatively dark portions of an image being shown on the display. Some prior devices address this issue by including an ambient light sensor or a user control that enables an increase in the brightness of the display when the ambient light level is high. This approach suffers from several limitations. One of those limitations is that quite often the light source, such as, for example, LEDs (light emitting diodes) or other light source, already is operating at maximum brightness and it is not possible to further increase their light output (sometimes referred to as brightness or intensity). Another limitation is that if increasing the brightness is possible, doing so actually may produce only a small increase in the legibility or contrast ratio of the image. This can be explained by the following example.
In the context of this example we make the following definitions.
A liquid crystal display (LCD) includes a backlight and a liquid crystal modulator. The backlight is a light source that illuminates the liquid crystal modulator. The liquid crystal modulator includes a number of pixel areas (sometimes, referred to as pixels) that can be operated to control light transmission through the respective pixels to create an image, for example.
Ambient lighting uniformly illuminates all pixels in the display. In the example here, for simplicity the brightness of the ambient is described as Low (level 1), Intermediate (level 8) or High (level 16). The Low level might correspond, for example, to typical office lighting conditions. The Intermediate level might correspond to indirect outdoor lighting. The High level might correspond to direct sunlight.
The backlight uniformly illuminates all pixels in the display. The brightness of the backlight is described as Minimum (level 1), Medium (level 2) or Maximum (level 3). The backlight described herein is one or more light emitting diodes (LEDs), but it will be appreciated that other types of light sources may be used and the it also will be appreciated that such illumination may be provided from a direction other than from the back or behind the liquid crystal modulator, e.g., from a side.
The LCD pixels can produce eight (8) shades of grey. Level 1 is the Blackest and level 8 is the Whitest. The use of grey levels in creating an image is, of course, known in the display art. The reference herein to the term “grey level,” “grey shade” and “shade of grey” may be used to mean the same; these terms are known in the art, as is mentioned elsewhere herein.
The “Grey Level Seen by the Viewer” applies to each pixel individually and is equal to the product of the (Grey Level of that pixel)×(the brightness of the backlight)+(the brightness level of the ambient). Values of the pixel Grey level seen by the viewer are described as between Darkest and Brightest.
Using these definitions, consider first an LCD that is presenting a “Normal” image. A “Normal” image occurs when the LCD utilizes all (or at least many) of the available shades of grey and the backlight level is Medium. Further, assume that the “Normal” image is viewed under Low ambient lighting conditions. This situation is illustrated in FIG. 1, which is briefly described just below.
FIG. 1 includes several portions A, B, C and D. FIG. 1, portion A is a graph illustrating the brightness level of ambient light from level 1 (dimmest) to level 16 (brightest); in the example represented by FIG. 1, portion A the brightness level of ambient light is shown as being at level 1. FIG. 1, portion B is the brightness level of the backlight used to illuminate the pixels of the liquid crystal modulator of the LCD; level 1 is the lowest brightness (dimmest), and level 3 is the highest brightness (brightest); in the example represented by FIG. 1, portion B the brightness level of the backlight is shown as being at level 2. FIG. 1, portion C is a graph representing grey level of eight exemplary respective pixels of the LCD; the grey levels range from level 1 (the blackest or darkest) to level 8 (the whitest or brightest) represented on the Y axis; eight respective pixels are shown spaced along the X axis representing the grey level of those respective pixels from grey level 1 through grey level 8. FIG. 1, portion D is a graph representing grey level seen by a viewer looking at the LCD, the respective grey level values being shown on the Y axis extending from seen grey level 1 (the darkest) to seen grey level 24 (the brightest); eight respective pixels are shown spaced along the X axis representing the grey level seen by a viewer looking at the LCD for eight respective pixels—those seen, grey levels extending from seen grey level 3 through seen grey level 17, as is described further below.
As is shown in FIG. 1, the viewer is presented an image (e.g., represented by the respective pixels in portion D) in which the eight pixels each present a different shade of grey. The shades of grey extend over a range of values from 3 to 17, that is, the range may extend over and utilize up to the 15 different grey levels that are represented in the graph of FIG. 1. The full texture of the image is presented and there is no contouring. The respective shades of grey seen by a viewer are determined by in a sense multiplying the brightness level 2 of the backlight (FIG. 1, portion B) times the grey level of the respective LCD pixel (FIG. 1, portion C, e.g., from grey level 1 to grey level 8, respectively), and ignoring the brightness level of the ambient light as having no impact because it is so low (e.g., shown as level 1 but possibly being at zero or in any event essentially having no impact on the grey level seen by a viewer).
Next, referring to FIG. 2, which is similar to FIG. 1, but here consider the “Normal” image is being viewed under Intermediate ambient lighting conditions, e.g. shown at level 8 in FIG. 2, portion B. As in the case of the example of FIG. 1, the viewer is presented an image in which the eight pixels as seen by a viewer each present a different shade of grey. The shades of grey extend over a range of values from 10 to 24, that is, the range may extend over and utilize up to the 15 different grey levels that are illustrated in the graph. As before, the full texture of the image is presented and there is no contouring. Note, however, that the brightest pixel now corresponds to the upper limit of the display, which is grey level 24 seen by the viewer. In FIG. 2 the respective shades of grey seen by a viewer (portion D) are determined by in a sense multiplying the brightness level 2 of the backlight (FIG. 2, portion B) times the grey level of the respective LCD pixel (FIG. 2, portion C, e.g., from grey level 1 to grey level 8, respectively), and then adding the brightness level of the ambient light, namely a value 8. Thus, for the first pixel to the left in FIG. 2, portion D, the grey level seen by the viewer is [(grey level of LCD pixel, a value of 1, portion C) times (brightness level of backlight, a value of 2, portion B)] plus (brightness level of ambient, a value 8, portion A) equals grey level 10 seen by viewer. Similarly, for the last pixel to the right in FIG. 1, the grey level value of 24 seen by the viewer is obtained by the just-described computation, e.g., [8 (grey level of LCD pixel)×2 (brightness level of backlight)] plus 8 (brightness level of ambient) equals 24.
In FIG. 3, the “Normal” image is viewed under Maximum ambient lighting conditions, e.g., level 16, as shown in portion A. Portions B and C of FIG. 3 are shown the same as in FIGS. 1 and 2. In the example of FIG. 3, as is seen in portion D, the viewer is presented an image in which only 4 grey levels are utilized. Five of the brightest pixels, e.g., as are shown as the five right-most pixels in FIG. 3, portion D, have reached the maximum grey level to be seen by the viewer. The shades of grey seen by a viewer extend from a value of 18 to 24, that is, 7 different levels are utilized. In this case, the full texture of the image is not presented and contouring is likely to occur.
The upper limit is a reflection of the fact that the human vision system has saturated and is not capable of further grey shade discrimination. Also, the upper limit may be a limiting characteristic of the LCD itself. In the drawings of this patent application, the upper limit is represented, for example, by a value number 24.